What causes this glitch in this mesh? How do I remove it?
I took a look at your file. First I tried switching to the smooth brush and putting the strength to 100 to try to smooth it out. It seemed to look better after but there was still something majorly wrong with it.

Then I checked it out in edit mode and noticed a naughty 5 sided NGON inside the mesh.

Ngons = bad bad bad. So I would fix that for a start. :)
Another thing I noticed while trying to give you a fixed version of the sculpt is that during your sculpting, you've managed to move the mesh off the centre of the grid, so now the sculpting is no longer symmetrical, so be casreful when moving your object and make sure the origin sits at 0.000 on the X axis at all times.

Now editing your sculpt in edit more when using the multi-resultion modifier can cause it to break your sculpt. (this is to do with the fact that all vertices are numbered and the multi-resolution modifier identifies the location of each individual vertex by it's assigned number). So if you delete a vertex and replace it, then the vertex numbering is all changed and it causes major problems with your sculpt. 

The best way to solve this is to first:

1) Go into Object Mode >> change the location of your sculpt to 0.000 on the X axis.

2) Go into Edit mode >> Remove your ngons (faces with more than 4 polygons) and try to make all your faces into quads (four polygon faces). I saw the back of the leg was an ngon and the upper legs contain two ngons each so there at least 5 ngons you can try to make into quads :)

Then press W and "Remove Doubles". Also, press Ctrl+N to make sure all your normals are facing outwards (these two are just in case).

3) Unfortunately, now you'll need to delete your multi-resolution modifier in Object Mode.

4) Now press Ctrl+A and Set Rotation and Scale. (Again this is just in case).

5) Now you'll have to start sculpting again. Yes, it's a pain in the backside but think of it as practice and usually when i have to do a sculpt twice, the second time always turns out better!

Good luck and happy sculpting!